1.1 Introduction

IT systems are becoming increasingly ubiquitous and help support most aspects of everyday life. The Internet has helped accelerate the rate at which IT is integrated into most social systems. People rely on IT systems to address most of their major human and social concerns such as health, education, entertainment, access to communication services, access to customer support, finances, safety, privacy, access to government services, and travel. The various concerns of individuals and of the society as a whole may face major breakdowns and incur high costs if IT systems do not meet the Quality of Service (QoS) requirements of performance, availability, security, and maintainability that are expected from them.

For example, a call to 911 the emergency number in the U.S. has to be answered by a dispatcher in a few seconds or human life may be endangered. When the stock market goes through periods of extreme ups and downs, a large number of online traders tend to flock to online trading sites, causing potential problems due to overloaded and non-responsive systems. The inability to trade in a timely manner may cause substantial financial losses. During health crises, such as the outbreak of new diseases, people need to get easy and fast access to health insurance companies to obtain authorization to be admitted to a hospital or to undergo a medical procedure.

In times of terrorism threats, major infrastructures, such as the telephone and cellular networks, may be targeted by terrorists or, in case of attacks to other structures, may become overloaded as their capacity to process calls is stretched thin, impairing the responsiveness of such systems. The operation of the military is becoming more and more dependent on an agile information and communications infrastructure to help locate, find, target, and destroy enemy forces. This infrastructure has to be properly designed and sized to handle the extraordinary demands of battlefield information exchanges.

Most people need to interact with automated or semi-automated customer support systems and expect near immediate response. Unfortunately, it is not uncommon for someone to be placed on hold for dozens of minutes before being connected to a human being who will take care of a problem or provide the needed information. These situations cause significant frustration and are a major cause for companies to lose customers.

The number of people signing up for access to a wide variety of communication services such as wireless and Internet access services is increasing at exponential rates. The growth in traffic has not been met by an adequate growth in system capacity. As a result, callers may hear the unpleasant recording "all circuits are busy, please try your call later," when trying to place a call. People have come to expect 24 / 7, instantaneous, and extremely reliable services.